A fuel cell comprises an electrolyte layer and a pair of electrodes placed on either side of the electrolyte layer, and generates electricity through an electrochemical reaction between fuel gas such as hydrogen and alcohol and oxidizing gas such as oxygen and air, which are supplied to the corresponding electrodes, with the aid of a catalyst. Depending on the electrolytic material used for the electrolyte layer, the fuel cell may be called as the phosphoric acid type, solid polymer type or molten carbonate type.
In particular, the solid polymer electrolyte (SPE) type fuel cell using an ion-exchange resin membrane for the electrolyte layer is considered to be highly promising because of the possibility of compact design, low operating temperature (100° C. or lower), and high efficiency.
The SPE typically consists of an ion-exchange resin membrane made of perfluorocarbonsulfonic acid (Nafion: tradename), phenolsulfonic acid, polyethylenesulfonic acid, polytrifluorosulfonic acid, and so on. A porous carbon sheet impregnated with a catalyst such as platinum powder is placed on each side of the ion-exchange resin membrane to serve as a gas diffusion electrode layer. This assembly is called as a membrane-electrode assembly (MEA). A fuel cell can be formed by defining a fuel gas passage on one side of the MEA and an oxidizing gas passage on the other side of the MEA by using flow distribution plates (separators).
Typically, such fuel cells are stacked, and the flow distribution plates are shared by the adjacent fuel cells in the same stack. When forming such a stack, it is necessary to seal off the passages defined on the surfaces of the MEAs from outside. Conventionally, gaskets were placed in the periphery of the interface between each adjoining pair of a MEA and a distribution plate. The contact area between the MEA and the gas diffusion electrode was ensured by pressing them together by applying an external force, typically with the aid of a suitable fastener. The required electric connection between the gas diffusion electrode and an electrode terminal connected to an external circuit was also ensured by pressing them together by applying an external force.
However, because the material used for the gas diffusion electrode, such as a carbon sheet, has surface irregularities, and the electrode terminal for connection with an external circuit is allowed to contact the gas diffusion electrode while providing flow paths for the fuel and oxidizer, the contact area between them is very much limited. Also, the SPE can function as an ion-exchange membrane only when impregnated with water, and the SPE when impregnated with water significantly changes its volume depending on the temperature. The flow distribution plates also expand and contract according to the temperature. The resulting stress affects the pressure that is applied to the fuel cell, and this prevents an accurate control of the pressure acting between the different layers of the fuel cell. In particular, it tends to prevent a reliable electric contact to be established between the electrode terminal and the gas diffusion electrode.
The carbon sheet is preferred as the material for the gas diffusion electrode, but cannot be made as thin as desired (in the order of a few μm) in view of the handling. This tends to undesirably increase the thickness of each fuel cell. Also, the carbon sheet is required to be porous, but the catalyst in the form of fine powder tends to fill the pores of the carbon sheet. Such a loss of porosity of the carbon sheet reduces the diffusion rate of the fuel gas and oxidizer gas which in turn reduces the efficiency of the device.